A prism is a transparent optical element with flat, polished surfaces that refract light. At least two of the flat surfaces must have an angle between them. The intersection of those surfaces is the prism apex, and the line of intersection is the apex axis. The prism face opposite the apex is termed the base. The exact angles between the surfaces can be configured based on the application. The traditional geometrical shape is that of a triangular prism with a triangular bottom and rectangular sides, and in colloquial use “prism” usually refers to this type. Some types of optical prism are not in fact in the shape of geometric prisms. Prisms can be made from any material that is transparent to the wavelengths for which they are designed. Typical materials include glass, plastic, and fluorite, although other materials are possible. Ray angle deviation and dispersion through a prism can be determined by tracing a sample ray through the element and using Snell's law at each interface.
A Fresnel prism is composed of prismatic elements arranged in an array with the base of each element adjacent to the apex of the next, yielding a device with refraction properties similar to a conventional prism of the same extent but significantly thinner and less bulky.
The shifted field of view through a conventional or Fresnel prism is substituted for the view behind the prism, resulting in a loss of field behind the prism at the prism apex equal in extent to the amount of shift provided by the prism (its angle of refraction). This field loss is referred to as the prism apical scotoma.